Wireless detection of motion

ABSTRACT

Disclosed herein are systems and methods for detecting motion of an object, such as the paw of rat. A tag comprising an inductive element and a capacitive element may be attached to the object and the motion detected by monitoring the affect of the tag on a time-varying magnetic field.

BACKGROUND

1. Field of the Invention

The invention relates to detection of motion. In some aspects, theinvention relates to detection of laboratory animal motion.

2. Description of the Related Art

In the discovery of neurological drugs, e.g., analgesics, many animalpain models are used. In one such model, a small amount of formalin isinjected into one rear paw of a rat. The resulting irritation causes therat to lick and/or to shake this paw in a repetitive, flinch-likemotion. This reaction is considered a painful response. A test compoundis administered to the rat after the formalin injection and changes inpaw motion counts are used to assess the compound's analgesic effect. Inother tests, a rat paw is exposed to a heat source and the motion of thepaw is monitored as the rats are exposed to various neurologicalcompounds. Such tests can be used to screen analgesic compounds or toperform other neurological experiments.

Currently, the most commonly used instrumentation for detecting rat pawmotion employs a metal detector to sense the motion of a metallicbracelet attached to a paw. With reference to FIG. 1, a metal detectortypically consists of two induction coils. A transmitter coil 100 isenergized with a relatively low-frequency (5-10 KHz) AC signal. Areceiver coil 110 is concentric and coplanar with the transmitter coil100. The coils are only weakly coupled through free space. When a smallpiece of metal (e.g., a rat paw bracelet 120) is introduced into thearea above the two coils, the coupling is increased and a strongersignal is detected by the receiver coils 110. The metallic bracelet 120concentrates the magnetic flux so that more field lines are captured bythe receiver coil 110, giving rise to a stronger detected signal. Asthis design relies on the phenomenon of flux concentration, braceletsmade from high magnetic permeability metals give rise to the largestsignals. Overall, this effect is weak and a very high level ofamplification is needed to detect it.

In a typical laboratory environment, this design suffers from severaldisadvantages. First, there is usually a significant amount of metal ina typical lab, often causing mistriggering of the device. For example,wristwatches worn by the operators are a common source of interference.Second, the receiver coil's 110 cross-section is large and a very highlevel of amplification is used causing stray magnetic fields fromvarious power sources to elicit false signals. Third, multiple detectorscannot be placed too close to each other because stray magneticinduction from one unit tends to falsely trigger its nearest neighbors.Thus, there is a need for improved motion detectors, particularly foruse in detecting rodent paw motion.

SUMMARY OF CERTAIN EMBODIMENTS

One aspect of the invention is a system for detecting motion of a tag,including a coil configured for generating a time-varying magneticfield, a tag comprising an inductive element and a capacitive elementpositioned with respect to the coil such that tag motion moves the tagrelative to the coil.

Another aspect of the invention is a system for detecting motion of atag, including a means for generating a time-varying magnetic field, ameans for sensing a change in the time-varying magnetic field, and atag, the tag comprising an LC circuit for causing a change in thetime-varying magnetic field when the tag is in motion within thetime-varying magnetic field.

Another aspect of the invention is a system for detecting motion of atag, including a sensing coil comprising a wire wound into a coil, thecoil defining a perimeter, an AC generator electrically coupled to thewire, a circuit electrically coupled to the wire, the circuit adapted todetect a change in AC current flowing through the wire, and a tag, thetag comprising an inductive element and a capacitive element, whereinthe tag is within the perimeter of the coil but not necessarily coplanarwith the coil, and wherein when the tag is in motion, the motion causesa change in any AC current flowing through the wire.

Another aspect of the invention is a tag for monitoring motion of a pawon a rodent, including an inductor and capacitor electrically coupled inparallel and a paw mount coupled to the inductor and capacitor.

Another aspect of the invention is a method of detecting motion of anobject, including exposing a tag comprising an inductive element and acapacitive element to a time-varying magnetic field, and detecting achange in the time-varying magnetic field caused by motion of the tagwithin the magnetic field.

Another aspect of the invention is a method of assaying for ananti-nociceptive drug, including attaching a tag to a paw of the rodent,the tag comprising an inductive element and a capacitive element,administering a potential anti-nociceptive drug to a rodent, exposingthe paw to a nociceptive stimulus, exposing the tag to a time-varyingmagnetic field, and monitoring motion of the paw by detecting a changein the time-varying magnetic field induced by motion of the tag withinthe field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art metal detecting apparatus used for detectingmotion.

FIG. 2 depicts a motion detecting apparatus including a sensing coil andan inductor-capacitor element tag.

FIG. 3 depicts a detection circuit for detecting change in atime-varying magnetic field.

FIG. 4 illustrates a rodent wearing a tag and placed within a sensingcoil.

FIG. 5 depicts a graph of the biphasic motion signal observed from ratpaw flinching.

FIG. 6 depicts an automatic pulse counting circuit.

FIG. 7 depicts a graph of the frequency of rat paw flinching with andwithout formalin.

FIG. 8 depicts a graph of the frequency of rat paw flinching in thepresence of formalin with and without morphine.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

In some embodiments, systems and methods are provided for detectingmotion of a tag. The tag may be attached to anything. In oneadvantageous embodiment, the tag is attached to a paw of a rodent suchas a rat. The tag may be attached to the paw using any suitablestructure. In one embodiment, a bracelet structure is used to attach thetag to a rodent's paw. In some advantageous embodiments, the tag doesnot need to have a power source and may be used as a wireless device. Inone embodiment, the tag comprises an inductive element and a capacitiveelement. Any electrical structures may be used for the inductive andcapacitive elements provided the electrical structures exhibitcorresponding inductive and capacitive behavior. In one embodiment, theinductive element is an inductor and the capacitive element is acapacitor. In one embodiment, the inductor and capacitor are connectedin parallel.

A tag including an inductor-capacitor element connected in parallel willcouple with a time-varying magnetic field. Thus, when aninductor-capacitor element is exposed to a time-varying magnetic field,the magnetic field may be altered by the presence of the tag.Furthermore, motion of the tag within the field may elicit atime-varying alteration of the field. Detection of this time-varyingalteration can provide an indication of the motion of the tag.Accordingly, in one embodiment, a system for detecting the motion of atag is provided that includes a time-varying magnetic field generator, amagnetic field sensor, and a tag including an inductive element and acapacitive element. The sensor may be used to detect a change in thetime-varying magnetic field induced by motion of the tag within thefield. In some embodiments, the magnetic field generator and magneticfield sensor may include wire coils. In one embodiment, a single coilmay serve as both the magnetic field generator and sensor.

FIG. 2 depicts one system for detecting motion of a tag. The systemincludes a tag 150 comprising an inductor and capacitor connected inparallel. The system also includes a sensing coil 152 connected to an ACsignal generator 154 and a detection circuit 156. The AC signalgenerator 154 sets up an alternating current through the coil 152, whichinduces a time-varying magnetic field. In one embodiment, the sensingcoil 152 is driven by an AC signal whose frequency (e.g., 0.5-1 MHz) ischosen to be close to or equal to the resonant frequency of theinductor-capacitor element in the tag 150. Because the drive frequencyis chosen to be approximately equal to this resonant frequency, theinductor-capacitor element is very efficient at removing energy from thesensing field. Thus, when the tag 150 is introduced into the sensingcoil's 152 time-varying magnetic field, the inductor-capacitor circuitabsorbs some of the field's energy. This energy loss can then be sensedwith the proper detection circuit 156 connected to the sensing coil 152.The time variation of the energy loss can be used to indicate motion ofthe tag 150 within the time-varying magnetic field induced by coil 152.

FIG. 3 depicts one embodiment of a detection circuit 156. The sensingcoil 152 forms one leg of a four-coil bridge 200, which is driven by ahigh-frequency AC signal from signal generator 154. The sensing coil 152is connected to the bridge 200 at nodes X and Y. The generator 154 isconnected to nodes Y and W of the bridge 200. When theinductor-capacitor element in the tag 150 is in the time-varyingmagnetic field induced by the sensing coil 152, the bridge 200 becomesincreasingly unbalanced, causing an increase in the output of adifferential amplifier 202 connected to the nodes X and Z of the bridge200. This output is AC-coupled by an output capacitor 203 and rectifiedby a rectifier 204 and amplified by a high gain amplifier 206. TheAC-coupled signal may then be passed through a low frequency bandpassfilter 208 so that only low frequency variation in the signal (e.g.,caused by motion of the tag 150 in the magnetic field) is detected.Those of skill in the art will recognize that many other circuit designsthan that described above may be used to detect magnetic field variationusing a sensing coil.

FIG. 4 depicts the sensing coil 152 with a rodent 220 placed within thecoil 152. The two terminals of coil 152 are connected to nodes X and Yof the four-coil bridge 200. The tag 150 comprising theinductor-capacitor element may be placed on a leg 222 of the rodent 220.When the rodent's leg 222 moves up and down, the tag 150 will also moveup and down, inducing a change in the current flowing through coil 152.

In some embodiments, because the signal produced by the apparatusdescribed above is AC-coupled, the low frequency variation in signalproduced by motion of an inductor-capacitor tag is indicative of rate ofmotion of the tag rather than absolute position of the tag. Thus,positive amplitudes indicate rate of motion in the upward direction andnegative amplitudes indicate rate of motion in the downward direction.FIG. 5 depicts a graph of signal amplitude generated by motion of a ratpaw flinching. Each flinch results in a biphasic signal. The positivelobe is systematically smaller in amplitude than the negative lobe,indicating that the rat puts down its foot slowly but lifts it quickly.

In some embodiments, a motion detecting apparatus may be automated. Forexample, the motion signal may be analyzed by a computer to determinethe number of up-down movements as a function of time. Thus, a systemmay be provided that provides an automated output of frequency of ratpaw flinching. For example, the system may count each biphasic signal asone flinch. FIG. 6 depicts one embodiment of an automated motioncounter. The output from the detection circuit 156, such as the circuitdepicted in FIG. 3, is input into op-amp 300, which outputs a pulse foreach biphasic signal from the detection circuit 156. The output fromop-amp 300 is input into pulse counter 302, whose output may be analyzedby data processing circuit 304. If desired, the resulting pulse countand/or frequency may be determined by data processing circuit 304 anddisplayed on output display 306.

In some embodiments, the motion detecting apparatus and methodsdisclosed herein have several advantages when compared with other motiondetecting systems. First, an inductor-capacitor element is veryefficient at coupling to a resonant magnetic field, thus generating alarge signal. Other structures in a typical laboratory environment areunlikely to be as efficient in coupling to the generated magnetic field.Thus, the chance of detecting extraneous signals is reduced. Second,given the resonant nature of an inductor-capacitor circuit, high levelsof amplification are not needed because the signal is fairly large.Thus, the system's sensitivity to interference from stray magneticfields in the environment is reduced. Third, inductor-capacitor elementscan easily be miniaturized using surface-mount components. For example,tags may be made to weigh very little (e.g., 0.3 grams). Thus, a rodentwearing the tag will not perceive significant inertia caused by the tag,which could interfere with its paw flinching. Fourth, several sensingcoils can function very close to each other without causingcross-interference. It was observed that two coils operating at the samefrequency did not interfere with each other even when placed only a fewinches apart. While not being bound by any particular theory, it isbelieved that coil-coil crosstalk is reduced because the amplificationlevel in each coil is not very high. It is believed that coil-tagcrosstalk is reduced because the tags only capture a very small area ofthe field of a sensing coil when not inside the circle of the coil.Accordingly, a large number of units may be deployed close to eachother. Such high-density deployment allows high-throughput usage such asfor testing multiple rodent paw flinching simultaneously.

In one embodiment, a method for assaying for an anti-nociceptive drug isprovided. A tag such as described above may be attached to a paw of arodent. A potential anti-nociceptive drug may then be administered tothe rodent. A nociceptive stimulus, such as a formalin injection or aheat source, may be applied to the paw of the rodent and the resultingmotion of the paw monitored by the above-described system.

Although one advantageous application of the systems and methodsdescribed herein is for detecting flinching of a rat paw. It should berecognized that the systems and methods may be used to detect motion ofany object placed within a sensing coil.

EXAMPLES Example 1

A rat was fitted with an inductor-capacitor tag on its left rear paw andplaced within a detector as described in FIGS. 2 and 3. A baseline ofpaw motion was detected. Formalin was then injected into the paw and theresulting signal was detected. FIG. 7 depicts a graph with each datapoint representing flinch frequency as function of time. The diamondsindicate the base line measurement and the squares represent the resultafter injection with formalin. The formalin results exhibit astereotypical two-phase regime. There was high frequency flinching inthe first 5-10 minutes. After a brief quiescence at about 10 minutes,prolonged and intensive flinching was detected for the next 30 minutesor so.

Example 2

A second rat received the same formalin treatment as the rat in Example1; however, an injection of morphine (3 mg/kg, sub-cutaneous) was alsoadministered. FIG. 8 depicts the flinching frequency of this second rat(triangles) compared with the first rat (squares). The second rat'sflinching pattern also displayed two phases but was much reduced interms of frequency when compared with the control (square data points),which did not receive any morphine.

What is claimed is:
 1. A system for detecting flinching motion of a footof an animal, comprising: a single sensing coil comprising a wire woundinto a coil, said coil defining a perimeter that surrounds the whole ofan animal; an AC generator electrically coupled to said wire; a circuitelectrically coupled to said wire, said circuit adapted to detect ACcurrent flowing through said wire; a tag attached to a foot of saidanimal, said tag comprising an inductive element and a capacitiveelement, wherein said tag is within the perimeter of said coil but notnecessarily coplanar with said coil, and wherein said circuitelectrically coupled to said wire, said circuit comprises an outputcapacitor providing an AC-coupled output indicative of rate of motion ofsaid tag rather than absolute position of said tag, said circuitproduces biphasic output pulses corresponding to flinching motions ofsaid foot, wherein the rate of tag motion is indicated by an amplitudeof each phase of the biphasic output pulse and an upward or downwarddirection of tag motion is indicated by a positive or negative polarityof each phase of said biphasic output pulse; and, a pulse counter countssaid output pulses from said circuit so as to count the number offlinching motions of said foot over a time period.
 2. The system ofclaim 1, wherein said tag comprises an inductor and a capacitorelectrically coupled in parallel.
 3. The system of claim 2, wherein saidAC generator is adapted to drive said sensing coil with an AC currenthaving a frequency, and wherein said electrically coupled inductor andcapacitor have a resonant frequency approximately equal to said ACfrequency.
 4. The system of claim 1, wherein said circuit comprises afour-coil inductive bridge, wherein said sensing coil comprises one legof said bridge.
 5. The system of claim 4, wherein said AC generator iselectrically coupled to two nodes of said bridge.
 6. The system of claim5, wherein said circuit comprises a differential amplifier electricallycoupled to two other nodes of said bridge.
 7. The system of claim 4,wherein said circuit comprises a differential amplifier electricallycoupled to said bridge.
 8. The system of claim 7, wherein said circuitcomprises a rectifier electrically coupled to said differentialamplifier.
 9. The system of claim 8, wherein the rectifier is directlycoupled to said differential amplifier.
 10. The system of claim 8,wherein said circuit comprises a high gain amplifier electricallycoupled to said rectifier and a low frequency band pass filterelectrically coupled to said high gain amplifier.